The present invention relates to an organic electroluminescent device (hereinafter may be called xe2x80x9corganic EL devicexe2x80x9d) and a method of manufacturing the organic EL device. More particularly, the present invention relates to an organic EL device suitably used for display apparatuses for home use or industrial use, light sources for printer heads, and the like, and to a method of manufacturing the organic EL devices.
Development of organic EL devices with an organic light-emitting layer inserted between electrodes of the devices have intensively been undertaken for the following reasons.
(1) Handling and production of organic EL devices become easy because the organic EL devices are complete solid elements.
(2) Organic EL devices do not require the additional luminous apparatuses because these devices can emit the light themselves.
(3) Organic EL devices are suitable for use with display apparatuses due to excellent visibility.
(4) A full color display can be easily provided using the organic EL devices.
However, because the organic light-emitting layer is an organic substance, injecting electrons from a cathode layer is not easy. In addition, because the organic substance generally can transfer electrons and positive holes only with difficulty, the organic light-emitting layer tends to deteriorate easily and produce leakage current when used for a long period of time.
Japanese Patent Application Laid-open No. 8-288069 discloses an organic EL device provided with an insulating thin layer between an electrode and an organic light-emitting layer as a means for extending the life of the organic EL device. The organic EL device disclosed in this patent application has a configuration in which an insulating thin layer of aluminum nitride, tantalum nitride, or the like is provided between an anode layer and an organic light-emitting layer or between a cathode layer and an organic light-emitting layer.
U.S. Pat. No. 5,853,905 discloses an organic EL device provided with an insulating thin layer between an anode layer and a light-emitting layer or a cathode layer and a light-emitting layer. The U.S. patent also discloses SiO2, MgO, and Al2O3 as materials for forming the insulating thin layers.
With an objective of providing an organic EL device at low cost without using expensive materials such as 4,4xe2x80x2,4xe2x80x3-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine (hereinafter may be abbreviated as xe2x80x9cMTDATAxe2x80x9d) and tetraaryldiamine derivatives, Japanese Patent Application Laid-open No. 9-260063 discloses an organic EL device having an inorganic material layer comprising NiO and at least one compound selected from the group consisting of In2O3, ZnO, SnO2, and compounds containing B, P, C, N, or O, or an inorganic material layer of Ni1xe2x88x92xO (0.05xe2x89xa6xxe2x89xa60.5) between an electrode and an organic light-emitting layer.
With an objective of extending the life of organic EL devices by reducing the energy difference between the work function of an anode layer and the ionization energy of a positive hole transport layer, Japanese Patent No. 2824411 discloses an organic EL device having an anode layer made of a conductive metal oxide material exhibiting a work function greater than indium tin oxide (ITO) such as RuOx, MoO3, and V2O5, for example. This Japanese Patent proposes in the specification an anode layer having a two layer structure consisting of these conductive metal oxide materials and an ITO to improve the light transmittance.
However, the inorganic compounds such as aluminum nitride, tantalum nitride, SiO2, and the like used as an insulating thin layer in the organic EL devices disclosed in Japanese Patent Application Laid-open No. 8-288069 and U.S. Pat. No. 5,853,905 have a great ionization potential which results in an increased driving voltage. Specifically, because the inorganic thin layers consisting of these inorganic compounds are electric insulating layers having an excessively large ionization energy, positive holes are injected from the anode layer by a tunnel effect. Therefore, a high driving voltage is required between the electrodes of the organic EL device to obtain a desired luminous brightness.
The organic EL device disclosed in Japanese Patent Application Laid-open No. 9-260063 is characterized by the use of NiO as a major component, which unduly limits the types of materials usable as an inorganic material layer and exhibits only a low luminous efficiency.
The organic EL device disclosed in Japanese Patent No. 282411 has the problems of small positive hole mobility and insufficient durability in spite of the use of the metal oxide materials such as RuOx (1xe2x89xa6xxe2x89xa62), MoO3, and V2O5.
In addition, the metal oxide materials such as RuOx (1xe2x89xa6xxe2x89xa62), MoO3, and V2O5 exhibit a large optical absorption coefficient of 27,000 cmxe2x88x921 or more, giving rise to remarkable coloration. Therefore, the light transmittance in the visible radiation range of the anode layer made of these metal oxide materials is very low, for example, about {fraction (1/9)} to ⅕ that of ITO, giving rise to problems such as a poor luminous efficiency and a small quantity of light which can be emitted.
In addition, even an anode layer with a two-layer structure consisting of lamination of a thin film of these metal oxide materials and ITO exhibits only a small light transmittance (about xc2xd that of ITO). Such an anode layer cannot be used in practice. Moreover, when an anode layer having such a two-layer structure is fabricated, the thickness of the ITO and a metal oxide film must be restricted within a prescribed range, resulting in a limitation in the manufacturing process.
The present inventors have conducted extensive studies to overcome the above problems and have found that, even in the case where an inorganic thin layer is provided in an organic EL device, an intermediate level for injection of electric charges can be formed in the inorganic thin layer by forming the inorganic thin layer from a combination of several specific inorganic compounds.
The inventors have further found that the combined use of specific inorganic compounds for forming the inorganic thin layer may produce an organic EL device with excellent transparency and durability, and superior luminous brightness at a low applied voltage (for example, less than DC 10V).
Accordingly, an object of the present invention is to provide an organic EL device having a specific inorganic thin layer and exhibiting excellent durability, a low driving voltage, and superior luminous brightness, as well as a method of efficiently manufacturing such an organic EL device.
Another object is to provide an organic EL device having an electrode layer made from a combination of specific inorganic compounds and exhibiting excellent durability, a low driving voltage, and superior luminous brightness, as well as a method of efficiently manufacturing such an organic EL device.
(1) One embodiment of the present invention (first invention) which is an organic EL device having an anode layer, an organic light-emitting layer, and a cathode layer is characterized by having a first inorganic thin layer formed between the anode layer and the organic light-emitting layer or a second inorganic thin layer formed between the cathode layer and the organic light-emitting layer, or having both the first and the second inorganic thin layers, wherein when the first inorganic thin layer is formed between the anode layer and the organic light-emitting layer, the intermediate level of the first inorganic thin layer is set at a value smaller than the ionization potential of the organic light-emitting layer; when the second inorganic thin layer is formed between the cathode layer and the organic light-emitting layer, the intermediate level of the second inorganic thin layer is set at a value greater than the electron affinity of the organic light-emitting layer; and an electric charge is injected through either the intermediate level of the first inorganic thin layer or the intermediate level of the second inorganic thin layer, or both.
This constitution enables charges to be injected with ease without utilizing a tunnel effect. Thus, an excellent organic EL device having a low driving voltage and exhibiting high luminous brightness and durability may be obtained.
The intermediate energy level in an inorganic thin layer will now be explained referring to FIGS. 7(a) and 7(b). FIG. 7(a) shows the relationship between an intermediate level in the inorganic thin layer on the anode layer side and an energy level in the organic light-emitting layer, and FIG. 7(b) shows the relationship between an intermediate level in the inorganic thin layer on the cathode layer side and an energy level in the organic light-emitting layer. In FIGS. 7(a) and 7(b), the intermediate level (Ei) of the first and the second inorganic thin layers is indicated between a valence band level (the ionization potential of the inorganic thin layer, Ev) and a conduction band level (the electron affinity of the inorganic thin layer, Ec).
Specifically, the intermediate level (Ei) in this inorganic thin layer is defined as the energy level which exists between Ev and Ec.
When a first inorganic thin layer is formed between the anode layer and the organic light-emitting layer in the present invention, the intermediate level (Ei) of the first inorganic thin layer is set at a value smaller than the energy level directed to the ionization potential (Ip) of the organic light-emitting layer, so that positive holes may be easily injected into the intermediate level (Ei) of the first inorganic thin layer from the anode layer as energy by applying a prescribed voltage. The positive holes then energetically move through the intermediate level (Ei) and are easily injected into the positive hole level (HOMO: Highest Occupied Molecular Orbital) in the organic light-emitting layer. This movement of the positive holes is schematically indicated by a dotted line in FIG. 7(a).
On the other hand, when a second inorganic thin layer is formed between the cathode layer and the organic light-emitting layer in the present invention, the intermediate level (Ei) of the first inorganic thin layer is set at a value larger than the energy level directed to the electron affinity (Af) of the organic light-emitting layer, so that electrons may be easily injected into the intermediate level (Ei) of the second inorganic thin layer from the cathode layer as energy by applying a prescribed voltage. Then, the electrons energetically move through the intermediate level (Ei) and are easily injected into the electron level (LUMO: Lowest Unoccupied Molecular Orbital) in the organic light-emitting layer. This movement of the electrons is schematically indicated by a dotted line in FIG. 7(b).
In either case, even if an inorganic thin layer is provided in the organic EL device, positive holes and. electrons easily move in the inorganic thin layer without using a tunnel effect, whereby the driving voltage is decreased and luminous brightness is increased.
In addition, durability may be remarkably improved by providing an inorganic thin layer like this.
Moreover, the first inorganic thin layer formed between the anode layer and the organic luminous body have the function such as an electric barrier which confines electrons in the LUMO level within the organic light-emitting layer due to the wide gap of the energy level.
In the same manner, the second inorganic thin layer formed between the cathode layer and the organic luminous body have the function such as a positive hole barrier which confines positive holes in the HOMO level within the organic light-emitting layer due to the wide gap of the energy level.
An organic layer other than the organic light-emitting layer may be formed between the first inorganic thin layer and the organic light-emitting layer in the present invention in addition to the organic light-emitting layer. In this instance, the intermediate level of the inorganic thin layer is set smaller than the ionization potential of this organic layer. Therefore, in such a case the intermediate level of the inorganic thin layer is not always necessarily smaller than the ionization potential of the organic light-emitting layer.
In the same manner, an organic layer other than the organic light-emitting layer may be formed between the second inorganic thin layer and the organic light-emitting layer. In this instance, the intermediate level of the inorganic thin layer is set greater than the electron affinity of this organic layer. Therefore, in such a case the intermediate level of the inorganic thin layer is not always necessarily greater than the electron affinity of the organic light-emitting layer.
The above-described intermediate level may be determined by measuring the fluorescence spectrum or electronic properties, and may be easily controlled by changing the materials used, for example.
(2) In the first invention, it is desirable that either or both of the first inorganic thin layer and the second inorganic thin layer contain at least one compound selected from the following group A and at least one inorganic compound selected from the following group B.
Group A: A chalcogenide of Si, Ge, Sn, Pb, Ga, In, Zn, Cd, Mg, Al, Ba, K, Li, Na, Ca, Sr, Cs, or Rb, and a nitride thereof
Group B: A compound of an element of Group 5A to Group 8 of the periodic table
The combined use of the group A and the group B ensures formation of an intermediate level in the inorganic thin layer. Because electric charges may be injected at low voltage through this intermediate level, the organic EL device not only possesses superior durability, but also exhibits high luminous brightness. In addition, the combined use of the group A and the group B does not impair transparency.
(3) In the first invention, it is desirable that the band gap energy of the inorganic thin layer (Ba) and the band gap energy of the organic light-emitting layer (Bh) satisfy the relationship xe2x80x9cBa greater than Bhxe2x80x9d.
Such a relationship improves transmittance of light and increases the quantity of light which is emitted out (an efficiency of light taken-out). In addition, such a relationship may increase the barrier characteristics against electrons or positive holes, resulting in an increase in luminous efficiency.
(4) Furthermore, in the first invention, preferably the group A is a chalcogenide of Si, Ge, Sn, Zn, Al, Ba, K, Li, Na, Ca, Sr, Cs, Rb, or Cd, or a nitride thereof.
Because these compounds may particularly maintain an excited state for a long period of time among the group A inorganic compounds, the organic EL device may exhibit a low light-quenching property and increase the quantity of light which is emitted.
(5) Furthermore, in the first invention, preferably the group B is an oxide of Ru, V, Mo, Re, Pd, or Ir.
The use of these compounds ensures formation of an intermediate level in the inorganic thin layer.
In addition, it is desirable that the inorganic thin layer containing these group B is provided between the anode layer and the organic light-emitting layer.
(6) Furthermore, in the first invention, preferably the content of the group B is in the range of 0.1 to 50 atomic % (hereinafter abbreviated as atm % or at. %) for 100 atomic % of the total of the inorganic thin layer.
This range of atomic % of the group B makes it easy to adjust the ionization potential of the inorganic thin layer, while maintaining a high transparency (light transmittance).
(7) Furthermore, in the first invention, preferably the thickness of the inorganic thin layer is in the range of 1 to 100 nm.
This range of thickness produces an organic EL device exhibiting superior durability, a low driving voltage, and high luminous brightness. In addition, the organic EL device does not become unduly thick if the thickness of the inorganic thin layer is maintained in this range.
(8) In another embodiment of the organic EL device of the present invention (second invention) which comprises an anode layer, an organic light-emitting layer, and a cathode layer, either the anode layer or the cathode layer or both comprise at least one compound selected from the following group A-1 and at least one compound selected from the following group B-1, or at least one compound selected from the following group A-2 and at least one compound selected from the following group B-2.
Group A-1: A chalcogenide of Si, Ge, Sn, Pb, Ga, In, Zn, Cd, Mg, Al, Ba, K, Li, Na, Ca, Sr, Cs, or Rb, and a nitride thereof
Group B-1: Inorganic compounds of an element of Group 5A to Group 8 in the periodic table and carbon.
Group A-2: A chalcogenide of Ge, Sn, Pb, Ga, In, Zn, Cd, Mg, Al, Ba, K, Li, Na, Ca, Sr, Cs, or Rb, and a nitride thereof
Group B-2: Inorganic compounds of an element of Group 5A to Group 8 in the periodic table, chalcogenide of Si and a nitride thereof, and carbon.
The combined use of the group A-1 and group B-1, or of the group A-2 and group B-2 may efficiently increase the ionization potential of the electrode layer. Specifically, the electrode layer has an ionization potential of 5.4 eV or more. Therefore, an organic EL device exhibiting durability, a low driving voltage, and high luminous brightness may be obtained.
In addition, the thus obtained electrode layer may exhibit excellent etching characteristics.
Furthermore, if the electrode layer contains a chalcogenide of Si or a nitride in at least one of the group A-1 and group B-1, or at least one of the group A-2 and group B-2, adherence between the substrate and the electrode layer is further improved and the electrode layer may be formed more uniformly.
When forming an anode layer using these inorganic compounds, it is desirable to set the work function at 4.0 eV or more taking into account injection characteristics of positive holes. On the other hand, when forming a cathode layer using these inorganic compounds, the work function is preferably set at less than 4.0 eV taking into account injection characteristics of electrons.
(9) Furthermore, in the second invention, preferably either the anode layer or the cathode layer or both has a specific resistance of less than 1 xcexa9xc2x7cm.
This is because high resistance of the electrode layer prevents uniformity of luminescence. Therefore, not only may injection characteristics of electrons and positive holes be improved, but also the driving voltage of the organic EL device may be more decreased by restricting the specific resistance in this manner.
On the other hand, if a material for forming electrode layers has a specific resistance of more than 1 xcexa9xc2x7cm, such a material may be used preferably for forming a two-layer structure in combination with another material having a specific resistance of less than 1 xcexa9xc2x7cm.
(10) Furthermore, in the second invention, the inorganic compound of group A-1 or group A-2 is preferably a chalcogenide of Sn, In, or Zn or a nitride thereof.
This is because, among inorganic compounds of group A-1 and group A-2, these inorganic compounds may have particularly small light-quenching characteristics.
Among these inorganic compounds, chalcogenides consisting of combinations of In and Zn are particularly preferred. Because the thin layer consisting of a combination of these inorganic compounds may have a flat surface, the film may have the stable amorphous properties. A thin layer made of a chalcogenide containing only In or In and Sn may have a surface less flat than that of the thin layer made from a chalcogenide containing In and Zn, because the former is crystalline or unstable amorphous.
(11) Furthermore, in the second invention, the compound of group B-1 or group B-2 is preferably an oxide (inorganic compound) of Ru, Re, V, Mo, Pd, or Ir.
The combined use of these inorganic compounds may make it easy to adjust the ionization potential and band gap energy in the electrode layers.
(12) Furthermore, in the second invention, preferably the content of the group B-1 or group B-2 is in the range of 0.5 to 30 atomic % for 100 atomic % of the total of the anode layer or the cathode layer.
This range of atomic % of the group B-1 or group B-2 may make it easy to adjust the ionization potential of the electrode layer, while maintaining a high transparency (light transmittance). In addition, the electrode layer thus obtained may exhibit excellent etching characteristics when an acid or the like is used as an etching agent.
(13) Furthermore, in the second invention, the anode layer or the cathode layer preferably has a thickness in the range of 1 to 100 nm.
This range of thickness may produce an organic EL device exhibiting superior durability, a low driving voltage, and high luminous brightness. In addition, the organic EL device does not become unduly thick if the thickness of the electrode layer is maintained in this range.
(14) Furthermore, in the first and the second inventions, the organic light-emitting layer preferably contains at least one of the aromatic compounds having a styryl group represented by the following formulas (1) to (3). 
wherein Ar1 is an aromatic group having 6 to 50 carbon atoms, Ar2, Ar3, and Ar4 are individually a hydrogen atom or an aromatic group having 6 to 50 carbon atoms, provided that at least one of Ar1, Ar2, Ar3, and Ar4 is an aromatic group, and n, which indicates a condensation number, is an integer from 1 to 6. 
wherein Ar5 is an aromatic group having 6 to 50 carbon atoms, Ar6 and Ar7 are individually a hydrogen atom or an aromatic group having 6 to 50 carbon atoms, provided that at least one of Ar5, Ar6, and Ar7 has a styryl group which may have a substituent, and m, which indicates a condensation number, is an integer from 1 to 6. 
wherein Ar8 and Ar14 are individually an aromatic group having 6 to 50 carbon atoms, Ar9 to Ar13 are individually a hydrogen atom or an aromatic group having 6 to 50 carbon atoms, provided that at least one of Ar8 to Ar14 has a styryl group which may have a substituent, and p, q, r, and s, which indicate condensation numbers, are individually 0 or 1.
(15) Another embodiment of the present invention (third invention) is a method for manufacturing any one of the above-described organic EL devices, which preferably comprises forming at least one layer in an organic electroluminescence by either a sputtering method or a vacuum deposition method, or both, by using a rotation evaporation apparatus capable of simultaneous evaporation.
This method ensures production of a thin layer with a uniform ratio of components even if a plurality of compounds is used, which consequently enables efficient manufacture of organic EL devices exhibiting high luminous brightness at a low driving voltage, and having excellent durability.
(16) In the third invention, preferably the inorganic thin layer is formed by a sputtering method and the organic light-emitting layer is formed by a vacuum deposition method.
This method of forming ensures production of an inorganic thin layer and an organic light-emitting layer having a precise and uniform thickness. Therefore, an organic EL device which has a uniform luminous brightness may be provided.